There have been many attempts in the prior art to improve upon the mechanical configuration for nuclear fuel assemblies. It is important in the design of such assemblies that premature wear of the mechanical structure due to coolant flow be minimized, and that the design is able to withstand thermal stresses. It is also important in such nuclear fuel assemblies that the flow of coolant about the fuel rods not be restricted, and that any debris within the coolant be trapped in a manner not restricting coolant flow. Even with the demanding design parameters for nuclear fuel assemblies, it is also important that the number of components associated with the structure be minimized for enhancing reliability, and reducing cost.
There are many examples of nuclear fuel assembly designs in the prior art that include some form of tie plate or tie plates within the assembly, for both supporting the fuel rods therein and providing rigidity to the structure. A number of such prior nuclear fuel assemblies are briefly described below.
Yates et al., U.S. Pat. No. 5,030,412, having the same assignee as the present invention, shows a debris screen for a nuclear fuel assembly. In FIG. 8 thereof a lower tie plate 30(30') is shown that incorporates a debris screen having a configuration as shown in FIG. 7. The debris screen is secured into a bottom portion of the tie plate 30(30'). Guide tube and instrument tube bushings must be provided separately for use in tie plate 30(30'). The present invention overcomes this disadvantage by providing integral guide tube bosses in the associated lower plate for the location of guide tube cap screws. Also, the present invention provides a lower tie plate produced from an integral casting configured for receiving a debris screen in a top portion thereof.
Bryan et al., U.S. Pat. No. 5,071,617, shows a nuclear fuel assembly that includes a cast lower end fitting 20. The end fitting 20 includes a top plate or a grid 22 configured with a plurality of rectangular through holes 24 for manifolding coolant through the fuel assembly. One centrally located hollow boss 26, and four hollow bosses 26 are included thereon at individual corners thereof, for receiving guide tubes 30(30'). Also, at opposing corners of the plate 22, leg members 28 are included for supporting the fuel assembly. Unlike the present invention, no provision is made in the lower end fitting 20 for receiving a debris screen in the top portion thereof. The present invention provides a lower tie plate casting configured for receiving a debris screen in the top portion of the tie plate, with the design of the top portion including a ribbing design configured to provide a strong support for the debris screen. The ribbing is formed integral with a webbing between associated integral guide tube bosses for substantially maximizing the strength of the frame without adversely restricting flow. Bryan et al. does not show such a structure for a lower tie plate, nor does Bryan et al. teach the elimination of separate guide tube and instrument tube bushings, as provided by the present invention.